Space Pirates is a science fiction trope that just won't go away. The image of pirate freebooters on the high seas is just too romantic for words, science fiction writers can't resist. Alas, in a scientifically accurate world, they are more or less impossible, much like space fighters and for similar reasons. There ain't no stealth in space, so it is practically impossible for a fat space galleon to be surprised in mid trip by a sinister space corsair flying the Jolly Roger. Or a rude surprise for a space merchant ship whose trajectory passes too near the Somali Asteroids for that matter. It would be several orders of magnitude easier for the "piracy" to take the form of grand theft from the merchant's warehouses on the ground.
Synonyms for "pirate" include corsair, buccaneer, and freebooter.
Do keep in mind that back in the days of Blackbeard piracy was punishable by death. If nothing else many pirates were savage murderers. The skull-and-crossed-bones flag contained skulls and bones as a message to the hapless galleon. The message was to hand over your treasure with no resistance, or the pirates would kill you and take it anyway. These are not the jolly comedic figures many of us remember from childhood stories.
And this is not just historical pirates. In the present day pirates operating around places like Indonesia will often torture and kill those who they capture.
But over and above the homicide aspect, under Admiralty law pirates were considered Hostis humani generis (Latin for "enemy of mankind"). The high seas could be claimed by no nation, they were the common property of all. So piracy was seen as a crime committed against all nations. Therefore all nations were bound by admiralty law to capture, bring to trial, and (if found guilty) execute any pirates they encountered; regardless of whether the nation had been attacked by that particular pirate or not. The trial usually was in a court martial held land-side, but in extreme cases could be by drum-head court-martial on the high seas convened by the officers of the capturing ship. Convicted pirates were traditionally hanged, in space I suppose they'd be thrown out an airlock without a space suit ("death by spacing").
The other class of seafaring criminal who were considered hostis humani generis were slavers.
Nowadays things are a bit different. Vessels on the high seas are under the protection of and in the jurisdiction of the vessel's flag state. Piracy is considered an offense of universal jurisdiction, so any state can board and seize a vessel engaged in piracy. And any state may try and impose penalties according to that state's laws.
Piracy does not work very well in a Rocketpunk solar-system based fictional universe. But if one is creating a fictional universe with faster-than-light starships, the author can tweak the properties of the FTL drive in order to allow piracy. As a matter of fact, many tweaks that will allow interstellar combat could also allow interstellar piracy.
Generally the pirates will have to perform naval boarding in order to loot the target merchant ship. In the Traveller RPG, boarding parties and the defenders use cutlasses since shooting bullet holes in a pressurized hull is considered to be a Bad Thing. But actually, all romanticism aside, it would take about an hour before the air leaking out a bullet hole depressurized the compartment to dangerous levels. So ditch the sword and take a submachine gun instead (long rifles are unwieldy in narrow spacecraft corridors).
As a general rule, merchant ships cannot be armed, armored, and combat crewed enough to fight off a pirate attack, not without increasing the amortized and operating cost and reducing the cargo capacity to the point where the ship cannot turn a profit. A "Q-ship" is a warship disguised as a merchant vessel, intended to fatally surprise hostile convoy raiders. They would also work against piracy. However since a Q-ship is a warship, it carries no cargo.
In Peter Hamilton's The Night's Dawn Trilogy, merchant starships can be armed yet still carry cargo with profit (by author fiat, i.e., because Peter Hamilton said so). The merchants tell everybody that the weapons are an anti-pirate measure (and because the ships sometimes hire out at mercenary warships). However some merchants ships actually commit piracy if they are sure no one will see or live to tell. In other words they are pirates hiding in plain sight.
As a side note, historically there was a bit of nuance used when employing Jolly Roger pirate flags.
A pirate ship would approach its merchant ship prey while displaying either no national flag ("no colors") or a false flag ("false colors") in the hope that the merchant captain was stupid enough to be fooled by this transparent ploy.
Once the pirate closed to firing range, it would fire a warning shot across the merchant's bow and hoist the black pirate flag. The message was "surrender without a fight, give us your cargo, and we'll let you live." Usually if the merchant fights but then surrenders, the pirates would tend to give them quarter if the black flag was flying.
If the pirate captain became angry (usually when the merchant refused to surrender), they'd hoist the bloody red flag. This means the merchant is in hot bubbling doo-doo up to their eyebrows. The red flag means the pirates are going to take the ship by force, and no quarter will be given. You had your chance Mr. Merchant, but you just had to go and cheese me off. Now we are going to capture your ship, kill you all, and take all your valuables. Enjoy your last few minutes of life.
If the merchant thinks the pirate is actually a privateer, the merchant might be inclined to resist. Privateers are forbidden to fight with no quarter, so the merchant knows they can surrender at any time during the battle. Of course the merchant is shafted if it turns out their assailant is a pirate after all. Or is a shady privateer captain who knows that Dead Men Tell No Tales, while making it clear this includes both the hapless merchant crew and any blabber-mouth members of his crew.
An implication of this is that the mere possession of a pirate flag is enough to convict one of the crime of piracy. Only a pirate would dare fly the Jolly Roger, as they was already under threat of execution
Some of these terms are universal, that is, they can easily be applied to your science fiction universe. Others are more specific to certain historical situations, but are still of interest to the science fiction author.
Pirates are pirates. They are people who rob people at gunpoint (and other violent crimes) for private ends and without authorization from any country; as long as this takes place on the high seas.
They are generally after valuable cargo carried by helpless merchant vessels. However some are also interested in kidnapping anybody worth a ransom, seizing the merchant ship itself, capturing the people on the ship in order to sell them into slavery, recruiting new pirates from the passengers, and committing other unspeakable acts.
Mild pirates just considered piracy their occupation. They just wanted the cargo with as little fuss as possible, and generally would not harm the merchant crew if they refrained from causing any trouble.
Brutal pirates on the other hand were bloodthirsty killers, torturing and slaughtering the merchant crew because they got a kick out of it.
Another term for pirate is "freebooter." This comes from the Dutch word "vrijbuiter", which comes from the word "vrijbuit" (plunder), which comes from "vrij" (free) + "buit" (booty).
When among only other of their kind, pirates would call themselves "pirates." If they were among Naval officers, judges, magistrates, snitches, and others who would hang them for their crimes, pirates would call themselves "merchant crewmen" or something equally innocuous.
Pretty much the only difference between a pirate and a privateer is a little scrap of paper called a Letter of Marque and Reprisal. This letter gave the privateers authorization from their home country to act in a piratey fashion on ships belonging to hostile countries, as spelled out in the letter of marque.
Some privateers would become pirates, attacking ship of countries not allowed by the letter of marque and hoping there were no witnesses. They would actually be pirates using the letter of marque to hide behind, pretending they were privateers.
When the war which prompted the letter of marque to be issued had ended, the letter was revoked. Some privateers would become pirates because they would ignore the revocation.
Privateers would call themselves “Gentleman of Fortune”, though that term was quickly stolen by full blown pirates.
Smugglers are just merchants who are trying to avoid legal restrictions on free trade. Local governments can be so unreasonable sometimes about which trade goods they deem to be illegal, subject to expensive import taxes, or only allowed to be carried by official government merchant ships in the official government monopoly. The same goes for hostile fleets laying siege to a planet, who take a dim view of blockade-runners supplying said planet with guns.
The smugglers are not blood-thirsty cut-throats stealing goods at gunpoint from captured merchant ships. Instead they are just peaceful merchants trying to quietly sneak past customs patrols and burdensome tariff regulations.
The government however view smuggling as piracy, that is, the smugglers are swiping money and goods out of what was intended to be a closed economic system. As far as the government is concerned, the end result is just as bad as if the smugglers were full-blown pirates.
Spain more or less owned the entire Caribbean until the Spanish Armada was defeated in 1588. After that Spain started to lose her colonies. A thorn in Spain's side was the French colony of Haiti on the island of Hispaniola.
The French settlers saw the plentiful hordes of wild oxen and pigs roaming the island, and the hungry Spanish ships frequently passing by, and so saw a business opportunity. The settlers would hunt the animals, cook the meat and sell it to the Spanish ships. The meat was cooked on racks called "boucans" (French for "barbecue"), so the Spanish called the French settlers "boucaniers." This was the origin of the term Buccaneers.
The Spanish later did something truly idiotic. Spain wanted to desperately hang on to its dwindling supply of Spanish colonies, and decided that the French buccaneers represented a threat to the security of the Spanish colony of Santa Domingo on the same island. Spain therefore came to the conclusion to forgo the delicious boucan-roasted meat and do their best to wipe out the French colony of Haiti.
The buccaneers took exception to this, and the Spanish quickly discovered that they had kicked a hornet's nest.
The buccaneers had no ships, but that was easily remedied. These guys typically would hunt the wild boars of Haiti armed only with two long daggers, they were mean and tough. On little canoes in the dark of night the buccaneers would stealthfully approach Spanish ships moored off the coast, quietly climb aboard, and silently cut the throat of the Spanish crew.
Thus equipped with ships, the buccaneers became fierce pirates. With the major difference that the buccaneers would only attack Spanish ships. They had a grudge against Spain, so to speak.
The buccaneers called themselves "The Brethren of the Coast", and had a long run in the Caribbean. They later established the pirate haven of Tortuga.
So the Spanish not only failed to eradicate the French colony of Haiti, they created a monster devoted to preying exclusively on Spanish ships. And they forever lost access to scrumptious Haitian barbecue.
Like most of the other major white-skinned seagoing European countries of the time, Spain committed the crime of enslaving brown-skinned people of African heritage. Some of the slaves would manage to escape and run away. The Spanish called such people "cimarron", which came from the Spanish word meaning "wild or untamed." They would create "cimarron settlements."
The moronic powers that be who ruled Spain has apparently learned nothing from the Buccaneer debacle. They had never heard the quote "insanity means doing the same thing over and over again and expecting different results."
The Spanish tried to wipe out the cimarron settlements the same way they tried to eradicate the buccaneers. And they got the exact same results.
In retaliation the cimarron targeted Spanish towns, and shipping. They soon acquired their own set of captured Spanish ships, called themselves "Marooners" after the term "cimarron", and started to prey on Spanish shipping. The term "marooner" came to mean any pirate with brown skin, but the original meaning was cimarron pirates with a grudge against Spain.
These were Muslim pirates and privateers based on the coast of North Africa who raided Christian towns and shipping all across the Mediterranean Sea, taking tons of ill gotten gains and about one million Christians who were sold as slaves. The Corsairs saw this as a religious duty to fight the infidels, though there was the opportunity to make a little money on the side by hijacking treasure.
The most famous were the Barbary Corsairs from the Barbary Coast of North Africa. They actually had letters of marque so were actually privateers. The Barbary Corsairs intercepted ships traveling through the Strait of Gibraltar or coming from the trading ports of Alexandria and Venice.
The situation, driving forces, and ecology of seagoing piracy on the Spanish Main is very different from interstellar buccaneers raiding merchant starships. Things will have to be altered in order for deep space piracy to make sense.
But some things are eternal. Pirates benefit from paid pirate-informants who spy around the starport and ferret out which merchant ship has the most lucrative cargo and when it is departing. Pirates will also need a fence or three to launder their ill-gotten gains into neutral money. Regardless of whether the pirates are operating in the 1600s or the 2500s.
Pirates will make heavy use of naval boarding, it is a large enough topic it has an entire section.
Another problem to be addressed if you want piracy to be viable is infrastructure. Captain Jack Sparrow's ship needed no fuel, only the winds. The crew can repair much of the ship if they can find an island that has trees. And there is no shortage of places that will accept gold coins and jewels. Now a pirate starship might be able to squeak by if they can use water or hydrogen for fuel, but it will be a real problem if their ships require antimatter or highly refined plutonium. Repairing ones ship is job for a shipyard, not a random asteroid with the crew frantically looking for nuggets of titanium. And fencing high tech computer chips will be a challenge. What pirates need is a Pirate Haven.
The two models of pirate havens are Tortuga and Port Royal. Tortuga was a place made by pirates and run by pirates for the benefit of pirates. Port Royal was a place that officially was against pirates. But unofficially they would purchase pirated goods, repair pirate ships, and show pirate crews a good time. In order to maintain the illusion of their anti-pirate stance the officials of Port Royal would strain themselves looking the other way, and never ever asking any embarrassing questions.
James Cambias said that manned piracy in space would require some kind of space habitat which is either run by a powerful Earth state yet tolerates piracy (the "no peace beyond the Line" model), or a space habitat which is essentially owned by pirates and can defend itself (the "Libertaria" model).
When is a pirate not a pirate? When they are a Privateer, of course. What's the difference? Not much, just a Letter of Marque and Reprisal. If a government is at war, and it doesn't want to spend a lot on warships and/or naval officers (or it wants plausible deniability), privateers are the solution. But the line between pirate and privateer is quite vague. The term "corsair" can mean either pirate or privateer.
Here's the deal. The privateer is a civilian warship, owned by citizens of the government. The owner receives a Letter of Marque from the government. The letter authorizes the privateer to attack vessels belonging to the "enemy" (as defined in the letter) in the name of the government. Other than what is stated in the letter, the privateer does not take any orders from naval command.
As the privateer captures or sinks enemy vessels, they get prize money. The privateer submits claims for bounty to the Prize Court to get the prize money. Generally they submit their claim by putting a prize crew in charge of the captured ship at the site of the battle and having it set sail to the port city containing the prize court. The prize money comes from the sale of the captured ship and its cargo (the government can purchase the captured ship at cost).
The prize crews usually are unhappy, since once they are off the privateer they do not receive any prize money from future captures. Only the crew on board the privateer during the battle get prize money from that battle. So the longer a crew person stays on the privateer, the more prize money they get.
The privateer is initially funded by private investors, who in exchange get a portion of the prize money earned. The privateer's officers and crew get the rest of the prize money. The government does not have to pay any money (prize money comes from sale of captured ship), yet gets the benefit of pressuring enemy convoy fleets and warships. It is a pretty good deal all around, as long as the privateer can regularly capture enemy vessels.
Of course the privateer is stuck with the bill for repairing any damage their ship sufferers during battle. And they run the risk of being captured or killed if the enemy ship turns out to be more than they can handle.
If the privateer commits certain offences, the navy can revoke the letter of marque as punishment. If the privateers mistakenly capture a ship of the wrong nationality, the prize court can order the captured ship returned the owners, and will not pay any prize money. In addition, the privateers will be liable to the owners for damages. If the privateers are smart, they will post a performance bond before hand, as insurance to pay for damages to owners. Legally privateers are not pirates, but warships. Pirate law does not apply, but the laws of naval warfare do.